In a medical field, an endoscope (a fiber scope or an electronic scope) is generally known as a device for observing a body cavity of a patient and has been practically used. Among endoscopes of this type, there is an endoscope provided with a variable power optical system having the variable power function for conducting fine observation for lesions.
For example, Japanese Patent Publication No. JP4819969B (hereafter, referred to as a “patent document 1”) describes a concrete configuration of a variable power optical system for an endoscope. The variable power optical system for an endoscope described in the patent document 1 includes a first lens group having a negative power, a second lens group having a positive power and a third lens group having a positive power which are arranged in this order from the object side, and is configured to perform focusing by moving the second lens group in accordance with change of the object distance without changing the entire length defined as a distance from the first lens group to the image plane.